The present invention generally relates to a cooling system for a water-cooled internal combustion engine and more particularly, to a cooling arrangement which is adapted to properly control temperatures of cooling water according to the state of operations of the internal combustion engine.
Conventionally, as a cooling system of an internal combustion engine, there has been widely employed a cooling arrangement of a cooling water circulating type which is so arranged that cooling water at a low temperature is supplied into a water jacket provided around cylinders of the engine, while the cooling water heated up to a high temperature by cooling said cylinders is fed to a radiator so as to be again supplied into the water jacket after having been cooled to a low temperature by said radiator.
In the known cooling arrangement as referred to above, there is provided an on-off valve, for example of a thermostat type at an engine outlet portion in a circulating passage of the cooling water for controlling the circulating amount of the cooling water by increasing or decreasing the opening degree of said on-off valve, thereby to maintain the cooling water temperature in the engine outlet portion at a predetermined value. However, in the control of cooling water temperature as described above, there are problems related to the so-called delay in response and hunting, resulting in inconveniences as follows. Specifically, in order to reduce the delay in response, it may be so arranged as to rapidly increase the circulating amount of the cooling water by accelerating the functioning speed of said on-off valve in the case where the cooling water temperature is higher than the predetermined value. In the above practice, however, for example, when the temperature of the cooling water flowing into the engine is low as affected by atmospheric temperatures, etc. or when the flow rate of the cooling water to be varied according to engine revolutions is high the engine is rapidly cooled, thus causing the temperature of the cooling water to undershoot below the predetermined value to a large extent, with a consequent generation of the undesirable hunting in a large amplitude. Meanwhile, for reducing the hunting, the functioning speed of the on-off valve referred to earlier may be lowered, but in that case, the delay in the response in control becomes conspicuous, requiring a long period of time for a high cooling water temperature to be lowered to a predetermined value, and therefore, there is a possibility that the engine is subjected to over-heating when it is under the state of a high load, with a large heating value.
Incidentally, with respect to control for cooling water-cooled engines, there has conventionally been proposed, for example, in Japanese Laid-Open Patent Publication Tokkaisho No. 57-168017, a cooling and control apparatus for water-cooled internal combustion engines. With attention directed to the problem that, even when the cooling water temperature is controlled to be constant, since the heating value of the engine varies depending on the state of operations, the engine is brought into an over-cooled state during small load periods with less heating value, the prior art cooling and control apparatus includes a cooling restricting device such as a flow rate control valve and the like for restricting a cooling capacity of the cooling system, a sensor for producing a signal correlated to temperatures at cylinder walls, and a control circuit for driving said cooling restricting device according to the output of said sensor, whereby the cooling restricting device is controlled by said control circuit based on a table predetermined according to the state of operations or the cooling restricting device is subjected to a feed-back control so that the cylinder wall temperature reaches a predetermined value by directly detecting such cylinder wall temperature. In the above prior art, however, a delay in response is also present before the cooling water temperature and/or cylinder wall temperature reaches the predetermined value after starting of said cooling restricting device, and there is also involved the problem as referred to earlier that, for decreasing the delay in response, the hunting phenomenon tends to become conspicuous in the case where the flow-in cooling water temperature is low or where the flow rate of the cooling water is high.